Two Demersal Trawl Surveys in the Gulf of Alaska: Implications of Survey Design and Methods

Abstract

In 1984 and 1987, demersal trawl surveys of fish stocks in the western and central Gulf of Alaska were executed by the National Marine Fisheries Service in cooperation with the Fisheries Agency of Japan. Details of the survey design and sampling methods are presented. Fishing power differences resulted from differences among vessels, gears, and trawling procedures. Chartered fishing vessels from the United States and Japan performed the sampling using modified commercial trawls as sampling gear. Nets used by the U.S. vessels were standardized to produce data appropriate for estimation of indices of fish abundance. In both surveys the nets used by the Japanese vessels departed markedly from that standard. Methods of estimating those fishing power differences, and applying the estimates to calibrate catches to a standard, have evolved through varied and conflicting strategies. To decide the most appropriate strategy for estimating and applying Fishing Power Corrections (FPCs). we defied a notion of negligibility. Based on this notion, final estimates of FPC were developed. Introduction Methods . . . . Methods Common to Both Surveys The Sample Unit Sampling Gear . . Taking the Sample . . Measuring Effort . . Survey Design . . Allocating Samples in Multispecies Surveys Vessels . . . . . Executing the Survey . . . Processing the Catch . . . Estimation . . . . . Methods Peculiar to 1984 . . . . Stratification and Survey Design . Vessels and Gear . . Net Mensuration . . . . Collecting Data to Estimate Fishing Power Corrections Executing the Survey . . . Methods Peculiar To 1987 . . . . . Stratification and Survey Design . Vessels and Gear . . . . . Net Mensuration . . . . . C o l l e c t i n g D a t a t o E s t i m a t e F i s h i n g P o w e r C o r r e c t i o n s : V